The idea for white is to check along g-file using his rook!
White to move and draw!
1. Nh6!!! Rxa7 and now White has perpetual check along the g-file! Draw. The idea is black's king can avoid a perpetual from g-file by white's rook even if his king move toward the rook. It is because the rook is safely protected on g-file. Two more obvious moves is
a) taking the knight with 1... Rxh6 also draw after 6.a8=Q Rxa8 2. Rxa8 Rxh5+ 3. Kg3 and black pawn will drop 3...Rg5+ 4. Kf4 Rg2 5. Rxa3 draw
b) ignoring white move and push his a-pawn still draw because black can't avoid perpetual by white's rook for e.g 1
... a2 2. Rg7+ Kc6
3. Rg6+ Kb7 4. Rg7+ Ka8 5. Rg8+ Kxa7 white still draw even after his a-pawn being captured! 6. Rg7+ Kb6 7. Rg6+ etc. Black's king can't escape the check
2. Rg7+ Kc6 3. Rg6+ Kb5 4. Rg5+ Kc4 5. Rg4+ Kd3 6. Rg3+ Ke2 7. Rg2+ Kf3 8. Rg3+ Kf4 9. Rg4+ Ke3 1/2-1/2
The problems is compose by Yugoslav chess composer - Djaja. You can read more about the problem above at chessbase.com